漂白剤による色素の退色機構 (第1報)

フェノール性ヒドロキシル基を一つ持つ色素の反応速度論的解析

抄録

A pseudo-first-order reaction constants (k_obs) of dyes having a single hydroxyl group inthe molecule (Orange I, Orange II) by various bleaching agents were measured as a function ofpH. Used bleaching agents were hydrogen peroxide, peracetic acid, potassium permonosulfate, and sodium hypochlorite. The k_obs of each dye showed maximum at intermediate pH betweenan acid dissociation constant of bleaching agent (pK_a) and an acid dissociation constant of dye (pK_d). Distribution curves of k_obs on pH showed symmetrical narrow distribution when the value of pKd was close to pKa, and symmetrical broad distribution when the value of pK_d was far apart from pK_a. These results were analyzed with reaction kinetics by assumming the ionic reaction among protonated and deprotonated form of the bleaching agents and the dyes. It wasproved that the distribution curves of k_obs on pH follow the following equation.
k_obs= [A] _t/ (1+ [H⁺] /K_a) (1+K_d/ [H⁺])
where [A] _t is the total bleaching agent concentration, k₁ is a second-order reaction constant byassuming the reaction between protonated dye ; [DyeH] and deprotonated bleaching agents ; [A^-]. The calculated values obtained by above equation were in good agreement with the observed values. Therefore, it was concluded that the decoloration mechanism of the dyes by bleaching agents conforms to the ionic reaction among protonated and deprotonated forms of the bleaching agents and the dyes. We have named this decoloration mechanism as “the mutual ionic reaction mechanism”.